Gross Unrealized Losses and Related Fair Values for AFS Securities Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	$ 5,425.7
Securities, less than 12 months, gross unrealized losses	100.1
Securities, 12 months or more, fair value	1,275.6
Securities, 12 months or more, gross unrealized losses	57.5
Total, fair value	6,701.3
Total, gross unrealized losses	157.6
Securities, less than 12 months, fair value		$ 2,749.1
Securities, less than 12 months, gross unrealized losses		24.5
Securities, 12 months or more, fair value		1,354.7
Securities, 12 months or more, gross unrealized losses		32.6
Total, fair value		4,103.8
Total, gross unrealized losses		57.1
U.S. Government obligations
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	532.8
Securities, less than 12 months, gross unrealized losses	12.7
Securities, 12 months or more, fair value	390.1
Securities, 12 months or more, gross unrealized losses	20.3
Total, fair value	922.9
Total, gross unrealized losses	33.0
Securities, less than 12 months, fair value		447.8
Securities, less than 12 months, gross unrealized losses		4.4
Securities, 12 months or more, fair value		416.6
Securities, 12 months or more, gross unrealized losses		13.2
Total, fair value		864.4
Total, gross unrealized losses		17.6
Municipal bonds
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	543.2
Securities, less than 12 months, gross unrealized losses	8.2
Securities, 12 months or more, fair value	215.2
Securities, 12 months or more, gross unrealized losses	8.9
Total, fair value	758.4
Total, gross unrealized losses	17.1
Securities, less than 12 months, fair value		240.0
Securities, less than 12 months, gross unrealized losses		1.5
Securities, 12 months or more, fair value		267.3
Securities, 12 months or more, gross unrealized losses		5.1
Total, fair value		507.3
Total, gross unrealized losses		6.6
Foreign government obligations
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	189.9
Securities, less than 12 months, gross unrealized losses	2.1
Securities, 12 months or more, fair value	130.5
Securities, 12 months or more, gross unrealized losses	3.6
Total, fair value	320.4
Total, gross unrealized losses	5.7
Securities, less than 12 months, fair value		321.9
Securities, less than 12 months, gross unrealized losses		2.7
Securities, 12 months or more, fair value		72.2
Securities, 12 months or more, gross unrealized losses		2.0
Total, fair value		394.1
Total, gross unrealized losses		4.7
U.S. corporate bonds
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	1,355.7
Securities, less than 12 months, gross unrealized losses	32.1
Securities, 12 months or more, fair value	174.1
Securities, 12 months or more, gross unrealized losses	9.8
Total, fair value	1,529.8
Total, gross unrealized losses	41.9
Securities, less than 12 months, fair value		568.8
Securities, less than 12 months, gross unrealized losses		6.1
Securities, 12 months or more, fair value		207.3
Securities, 12 months or more, gross unrealized losses		3.6
Total, fair value		776.1
Total, gross unrealized losses		9.7
Foreign corporate bonds
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	656.7
Securities, less than 12 months, gross unrealized losses	12.7
Securities, 12 months or more, fair value	189.2
Securities, 12 months or more, gross unrealized losses	6.3
Total, fair value	845.9
Total, gross unrealized losses	19.0
Securities, less than 12 months, fair value		417.4
Securities, less than 12 months, gross unrealized losses		3.0
Securities, 12 months or more, fair value		159.4
Securities, 12 months or more, gross unrealized losses		3.5
Total, fair value		576.8
Total, gross unrealized losses		6.5
RMBS
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	887.3
Securities, less than 12 months, gross unrealized losses	21.3
Securities, 12 months or more, fair value	116.5
Securities, 12 months or more, gross unrealized losses	5.4
Total, fair value	1,003.8
Total, gross unrealized losses	26.7
Securities, less than 12 months, fair value		284.2
Securities, less than 12 months, gross unrealized losses		1.6
Securities, 12 months or more, fair value		131.5
Securities, 12 months or more, gross unrealized losses		3.0
Total, fair value		415.7
Total, gross unrealized losses		4.6
CMBS
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	277.9
Securities, less than 12 months, gross unrealized losses	4.6
Securities, 12 months or more, fair value	32.3
Securities, 12 months or more, gross unrealized losses	2.8
Total, fair value	310.2
Total, gross unrealized losses	7.4
Securities, less than 12 months, fair value		112.2
Securities, less than 12 months, gross unrealized losses		0.5
Securities, 12 months or more, fair value		34.7
Securities, 12 months or more, gross unrealized losses		1.8
Total, fair value		146.9
Total, gross unrealized losses		2.3
Other asset-backed securities
Available-for-sale [Line Items]
Securities, less than 12 months, fair value	982.2
Securities, less than 12 months, gross unrealized losses	6.4
Securities, 12 months or more, fair value	27.7
Securities, 12 months or more, gross unrealized losses	0.4
Total, fair value	1,009.9
Total, gross unrealized losses	$ 6.8
Securities, less than 12 months, fair value		211.1
Securities, less than 12 months, gross unrealized losses		0.9
Securities, 12 months or more, fair value		65.7
Securities, 12 months or more, gross unrealized losses		0.4
Total, fair value		276.8
Total, gross unrealized losses		1.3
Debt Securities
Available-for-sale [Line Items]
Securities, less than 12 months, fair value		2,603.4
Securities, less than 12 months, gross unrealized losses		20.7
Securities, 12 months or more, fair value		1,354.7
Securities, 12 months or more, gross unrealized losses		32.6
Total, fair value		3,958.1
Total, gross unrealized losses		53.3
Common Stock
Available-for-sale [Line Items]
Securities, less than 12 months, fair value		145.7
Securities, less than 12 months, gross unrealized losses		3.8
Total, fair value		145.7
Total, gross unrealized losses		3.8
Equity Securities
Available-for-sale [Line Items]
Securities, less than 12 months, fair value		145.7
Securities, less than 12 months, gross unrealized losses		3.8
Total, fair value		145.7
Total, gross unrealized losses		$ 3.8